American officials say they have confirmed that the Syrian government has used chemical weapons against the insurgency seeking to remove it, the Associated Press reports.

An internal memorandum circulating within the Obama administration has assessed that chemical weapons, most likely the nerve gas sarin, were used multiple times in battle against the Syrian rebels.

The “intelligence community assesses that the Assad regime has used chemical weapons on a small scale against the opposition multiple times in the last year,” according to that memo, as cited by The New York Times.

White House officials speculated over evidence that nerve gas had been used as of April, but that evidence is now being called “definitive” – with Congressional sources describing the conclusion as crossing the “red line” for US military intervention as previously defined by the president.